Glasses designed to attenuate the ultraviolet, violet, and blue regions of the electromagnetic spectrum have been commercially available for many years. Such glasses have generally contained microcrystals of cadmium sulfoselenides to filter out those regions. There are several features intrinsic to those glasses, however, which lessen their suitability for certain applications. To illustrate:
First, if the heat treatment to which the glasses are necessarily exposed to develop the microcrystals therewithin is not properly performed, the crystals grow to a size large enough to cause objectional scattering of visible light passing therethrough, thereby resulting in the development of haze in the glass.
Second, exposure of those glasses to ultraviolet radiation produces a bright visible light fluorescence, thereby creating unwanted optical noise.
Third, the absorption character of the cadmium sulfoselenide crystals changes rapidly with variations in temperature. For example, heating those glasses reduces the amount of visible radiation that is transmitted therethrough.
Fourth, all radiation having wavelengths shorter than the filter cutoff is very strongly attenuated. In certain applications it can be desirable to permit a small amount of short wave blue light to pass therethrough in order to improve the overall color rendition; that is, to make the colors viewed through the glass more "real". The ability to achieve that property is extremely difficult to achieve by varying the concentrations of the cadmium sulfoselenide colorants.
Fifth, it is desirable to avoid use of cadmium because of its toxic nature.
The virtual elimination of those undesirable features, while simultaneously developing a fixed color or tint therein without the inclusion of conventional glass colorants, has been accomplished in commercially marketed, silver halide crystal-containing, photochromic glasses. Thus, by subjecting a silver halide crystal-containing photochromic glass to a defined thermochemical treatment, a constant permanent color can be produced in an integral surface layer thereon.
The presence of microcrystals of the silver halides AgCl, AgBr, and/or AgI effects the reversible darkening displayed by a photochromic glass when sequentially exposed and removed from exposure to actinic radiation, such radiation customarily consisting of ultraviolet radiation. U.S. Pat. No. 3,208,860 (Armistead et al.) furnishes the fundamental explanation of the photochromic phenomenon demonstrated by glasses containing microcrystals of a silver halide. U.S. Pat. No. 4,190,451 (Hares et al.) discloses more recently developed photochromic glass compositions containing silver halide crystals which exhibit rapid darkening and fading characteristics.
U.S. Pat. Nos. 3,892,582 and 3,920,463 (Simms) provide examples of thermal reduction heat treatments operable in developing integral yellow surface layers in silver halide crystal-containing photochromic glasses, while U.S. Pat. No. 4,240,836 (Borrelli et al.) discloses a modified thermal reduction treatment of photochromic glasses of similar base compositions which enables the development of a broader range of colors therein.
The method disclosed in the last patent comprises subjecting the photochromic glasses to a heat treatment at a temperature not exceeding about 450.degree. C. for a time sufficient to generate certain visible light absorption characteristics in an integral surface layer in the article. The light absorption characteristics developed as a result of the thermoreduction heat treatment are such that the glass, in the undarkened state, demonstrates a spectral transmittance curve comprising at least one treatment-induced absorption peak having a location and an intensity such that the peak falls within the transmittance region to the right of line CB in FIG. 1 of the drawings. The peak is not present in the parent photochromic glass from which the surface-colored article is produced. Consequently, in the surface colored glass article the color in the surface layer differs from the color of the bulk glass, which condition can be readily determined by comparing the spectral transmittance characteristics of the article before and after the removal of a small amount of the surface layer therefrom.
The mechanism underlying the development of the surface coloring effects was theorized as being caused by the chemical reduction of silver in contact with silver halide microcrystals in a region very close to the surface of the glass article, with the identity of the color being determined by the geometric form and arrangement of metallic silver on those microcrystals. That theory was confirmed by means of experiments demonstrating that, when a specific thermal reduction treatment was applied to a photochromic glass, the glass displayed a variety of absorption peaks depending upon the process initially employed to generate the silver halide microcrystals in the glass.
Other patents disclosing methods for imparting and/or modifying integral colored surface layers in photochromic glasses containing silver halide crystals include U.S. Pat. No. 4,290,794 (Wedding), U.S. Pat. No. 4,537,612 (Borrelli et al.), U.S. Pat. No. 4,710,430 (Borrelli et al.), U.S. Pat. No. 4,832,724 (Borrelli et al.) and U.S. Pat. No. 4,840,655 (Borrelli et al.)
U.S. Pat. No. 5,381,193 (Wedding) is directed to the production of silver halide crystal-containing photochromic glasses wherein radiations having wavelengths shorter than about 550 nm are substantially attenuated, but which are designed to permit a small blue transmission so as to reduce distortion of color perception. The method for preparing such glasses involved subjecting the photochromic glasses to a specifically-defined time-temperature treatment in the range of 2-12 hours at a temperature in the range of 320.degree.-550.degree. C.
U.S. application Ser. No. 08/334,929, filed Nov. 7, 1994 by T. B. Havens et al. under the title LASER EYEWEAR PROTECTION, describes the production of silver halide crystal-containing glass articles which are essentially opaque to ultraviolet radiation and, optionally, may be opaque to radiation having wavelengths up to 550 nm. The method comprises subjecting silver halide crystal-containing glass articles which exhibit photochromic behavior to a hydrogen-containing atmosphere at a temperature between 375.degree.-500.degree. C. for a period of at least four hours to produce an integral reduced layer on the article having a sufficient depth to effectively prevent the transmission of ultraviolet radiation therethrough and, optionally, to prevent the transmission of radiation having a wavelength up to 550 nm. The application noted that the final article did not exhibit photochromic behavior. Hence, whereas the starting glass article exhibited photochromism, because the integral surface layer developed thereon blocks the passage of ultraviolet radiation there is no transmission of the actinic radiation necessary to react with the silver halide crystals to cause the glass to darken.
Whereas photochromic behavior can be effected in silver halide-containing glass articles as such are being cooled from a glass melt, that procedure does not permit careful control of time and temperature. Accordingly, the essentially universal practice for producing photochromic glass articles is to form a glass article of a desired configuration from a melt (frequently annealed), and thereafter to subject the article to a selected time-temperature heat treatment to cause the in situ growth of silver halide microcrystals. The temperatures of those heat treatments range from about 100.degree. C. above the annealing point of the glass up to about the softening point thereof for times ranging from several minutes to several hours, depending upon the temperature employed, shorter times being operable at higher temperatures.
As can be appreciated, that heat treatment process to develop silver halide microcrystals adds cost to the product. Moreover, in like manner to the filter glasses disclosed in Ser. No. 08/334,929, supra, there are applications wherein photochromic behavior is not needed and may even be undesirable. Therefore, whereas it is possible to prepare filter glasses designed to attenuate ultraviolet, violet, and blue regions of the radiation spectrum with or without a small blue transmission by means of the thermoreduction of silver halide crystal-containing photochromic glasses, such glasses have restricted applicability. For example, filter glasses are frequently employed in the fields of optometry and ophthalmology where the property of photochromism is undesirable and total attenuation of ultraviolet radiation is not required.
It was explained above that glasses operable for filtering the ultraviolet, violet, and blue regions of the radiation spectrum are commercially available. Because, however, those glasses have generally relied upon the presence of cadmium sulfoselenide crystals, they are subject to a number of undesirable features.
Accordingly, the principal objective of the present invention was to devise a method/glass composition operable to produce glasses which strongly attenuate radiations in the ultraviolet, violet, and blue regions of the spectrum, but which glasses would not exhibit photochromic behavior or contain cadmium sulfoselenide or other crystal phases rendering them subject to the undesirable features exhibited by glasses containing cadmium sulfoselenide crystals.